Quintarianism
Quintarianism would probably be a good fit for BattleMaster. It has built-in orders that would be pretty easy for people to fit into. Would be great of there were ways to create subgroups, or sects, inside existing religions. But the four associated orders (there's no associated order for the Bastard, right?) It also has the advantage of being from a popular fantasy series, so you'd probably find quite a few people willing to RP a system they enjoyed reading about.
It also has the advantage of having a built-in competing sect, the Quadrenes, that uses four of the same five gods, but considers the Bastard to be evil.
